Thermodynamically stable amyloid-β monomers have much lower membrane affinity than the small oligomers

Amyloid beta (A beta) is an extracellular 39-43 residue long peptide present in the mammal an cerebrospinal fluid, whose aggregation is associated with Alzheimer's disease (AD). Small oligomers of A beta) are currently thought to be the key to toxicity. However, it is not clear why the monomers of A beta, are non-toxic, and at what stage of aggregation toxicity emerges. Interactions of A beta with cell membranes is thought to be the initiator of toxicity, but membrane binding studies with different preparations of monomers and oligomers have not settled this issue. We have earlier found that thermodynamically stable A beta monomers emerge spontaneously from oligomeric mixtures upon long term incubation in physiological solutions (Nag et al., 2011). Here we show that the membrane-affinity of these stable A beta monomers is much lower than that of a mixture of monomers and small oligomers (containing dimers to decarners), providing a clue to the emergence of toxicity. Fluorescently labeled A beta(40) monomers show negligible binding to cell membranes of a neuronal cell line (RN46A) at physiological concentrations (250 nM), while oligomers at the same concentrations show strong binding within 30 min of incubation. The increased affinity most likely does not require any specific neuronal receptor, since this difference in membrane-affinity was also observed in a somatic cell-line (HEK 293T). Similar results are also obtained for A beta(42) monomers and oligomers. Minimal amount of cell death is observed at these concentrations even after 36 h of incubation. It is likely that membrane binding precedes subsequent slower toxic events induced by A beta. Our results (a) provide an explanation for the non-toxic nature of A monomers, (b) suggest that A beta toxicity emerges at the initial oligomeric phase, and (c) provide a quick assay for monitoring the benign-to-toxic transformation of A.